Types are now bidirectional

It is still possible to define types as functions (Buffer => string for 
state topics, string => Buffer for command topics). Otherwise types have 
from and to properties.
(Fixes #101)

types/types.js

@@ -10,15 +10,3 @@ declare type Classes = {
 declare type State = Map<string,string>;
 
 declare type Point = [number, number];
-
-declare type Layer = {
-  image: string,
-  name: string,
-  baseLayer?: boolean,
-  defaultVisibility: "visible" | "hidden",
-  opacity?: number,
-  bounds: {
-    topLeft: Point,
-    bottomRight: Point
-  }
-};